Hardboard, fiberboard, particleboard and similar boards are fabricated from wood fibers or chips and consolidated under heat and pressure. Prefinished hardboard is made by consolidating lignocellulosic fibers under pressure in a press followed by application of one or more primer coats and topcoats. The prepaint sealers of the instant invention are applied to the preformed board after removal from the press and before application of a paint primer or topcoat. The sealer may be applied at room temperature or before the board cools. In either case it is integrally cured into the fiberboard surface.
For water repellency it is known to bond the fibers together; for this purpose some manufactures treat the fiber sheet with a small amount of synthetic resin and wax. Notwithstanding such treatment, hardboards still lack good water repellency and have a high degree of water absorption. This water sensitive characteristic of hardboard causes deformation or swelling of the hardboard and failure of later applied coatings. It has been known that vinylidene chloride (VDC) copolymers possess high barrier property to oxygen, carbon dioxide, and water vapor. In the past, the major application of VDC copolymer was used in paper coatings, to provide paper of low moisture vapor transmission rate, e.g. U.S. Pat. No. 3,696,082; USSR Pat. No. 439,946; Ger. Offen No. 2,112,126; Ger. Offen No. 2,522,805; Jpn. Kokai Tokkyo Koho JP No. 57,117,698. Recently, it was employed as anticorrosive coating for metals. In U.S. Pat. No. 3,883,455, vinylidene chloride polymer latex was used as a non-bleeding coating for wood of a high phenolic content in the presence of an organic acid and buffering salt. More recent patents include U.S. Pat. No. 4,336,174 (water-soluble melamine/formaldehyde-styrene/acrylic); and U.S. Pat. No. 4,374,899 (Sanfilippo and White); and U.S. Pat. No. 4,201,802 (Vande Kieft) (vinyl alcohol/fatty acid-esters copolymer). British patent No. 1,558,411 teaches the preparation of various vinylchloride/vinylidene chloride copolymer aqueous latex which are useful for metal coatings and paints generally. These U.S. patents relate mainly to "pre-press" sealers as distinguished from the "prepaint" sealers of the present invention. Pre-press sealers are applied before the fiber board is compacted under heat and pressure. In contrast, prepaint sealers are applied after the board is compacted. The board treated with prepaint sealer is subjected to an oven or infra-red bake before a primer paint and paint topcoat is applied.
The above-described defects, including water sensitivity, have been overcome by the prepaint sealers of the present invention prepared from vinylidene chloride latices. In addition to improved board properties, these boards have excellent water repellency and maintain dimensional stability even when exposed to humidity and outdoor conditions.